A well system (e.g., an oil or gas well) can include a wellbore that is typically drilled for extracting hydrocarbons from a subterranean formation. Various sensors can be positioned in the wellbore for detecting well system characteristics, such as temperature, pressure, sound level, the presence of a fluid, or the physical state (e.g., solid, liquid, or gas) of a substance (e.g., cement) in the wellbore. In some examples, the sensors can transmit data to a well operator (e.g., at the well surface). The well operator can rely on the data to determine if the well system is safe, compliant with particular standards, contains anomalies, or has other characteristics of interest.